The present invention relates to a method for processing chromium oxide-containing substances, for example, stainless steel slag as discharged in refining stainless steel, chromium slag as discharged in producing chromium compounds such as sodium bichromate, slag of molten wastes, sewage sludge, slag of molten sewage sludge and the like, thereby to prevent the release of Cr6+ from those substances.
Stainless steel slag as discharged in refining stainless steel, and chromium slag as discharged in producing chromium compounds such as sodium bichromate, contain chromium oxides at a level of a few %. A part of those chromium oxides is often oxidized to Cr6+, depending on the working conditions, and Cr6+ is released from them.
Where stainless steel slag and chromium slag are used as materials in roadbeds, materials in temporary works, fillers in civil engineering reclamation works and the like, it is indispensable that Cr6+ is not released from the slag.
Recently, recycling of slag as formed by melting ashes from garbage incinerators, sludge and the like in roadbeds, tiles and the like is now under consideration in the industry. However, the slag formed from some types of ashes from garbage incinerators, sludge and the like will often release Cr6+ to make it difficult to recycle the slag.
Accordingly, for safely recycling such slag in roadbeds, fillers in civil engineering reclamation works and the like, there have been proposed some new techniques of preventing the release of Cr6+ from the slag.
For example, for preventing the release of Cr6+ from stainless steel slag, there has been disclosed a method of adding aluminum ash and magnesia-based industrial wastes to stainless steel slag (see Japanese Patent Application Laid-Open (JP-A) No. Hei-6-171993).
When industrially carrying out the disclosed method, the intended aluminum ash and magnesia-based industrial wastes are previously introduced into a slag-receiving kiln, into which molten stainless steel slag is poured, stirred and mixed by the energy of the falling slag melt.
In this method, however, the slag melt is highly viscous, and as to this is different from molten steel, and it is difficult to produce a homogeneously mixed condition by the stirring and mixing means assisted only by the energy of the falling slag melt, resulting in the failure to completely stabilize all chromium oxides in the slag to thereby prevent the release of Cr6+ from the slag.
Another method was proposed, of adding a divalent S compound such as FeS or the like to molten slag resulting from decarburizing and refining stainless steel. The resulting slag is then reduced to recover Cr in the slag. The slag is then stirred with an inert gas stream introduced thereinto, thereby making the slag have an S content of not less than 0.20 % by weight so as not to release Cr6+ therefrom (see JP-A Hei-8-104553).
In this method, an inert gas stream is introduced into the slag melt to homogeneously mix the slag melt with the additive, thereby stabilizing the chromium oxides existing in the slag. This method is effective in preventing the release of Cr64  from the slag, but requires the troublesome operation of introducing such an inert gas stream into the reduced slag melt while using the additive for lowering the viscosity of the slag melt. Therefore, the method is not economical.
In addition, the method is further troublesome in that, if the additive is added to the slag melt in the refining furnace in order to ensure the homogeneous mixing of the slag melt with the additive, the additive will contaminate the steel melt.
On the other hand, for preventing the release of Cr6+ from chromium slag as discharged in producing chromium compounds, a method has been employed of reductionally roasting the slag to thereby reduce Cr6+ to Cr3+ for rendering the slag harmless. Economically, however, this method is troublesome in that it requires a roasting step.
Sewage sludge is incinerated to reduce its volume prior to being used for reclamation. To prevent the release of Cr6+ from the incinerated ashes, a method is used for controlling the air ratio during incineration to be less than 1 (see Journal of the Drainage Works Association of Japan, Vol. 38, No. 378, pp. 29-32, 1994).
However, as pointed out in this publication, it is extremely difficult to ensure the optimum operation for a variety of sludges derived from various sites and having different properties, and the same applies also to the incineration of industrial wastes.
To reduce Cr6+ in chromium oxide-containing substances at room temperature, there is generally employed a method using ferrous sulfate as the reducing agent. In this method, however, it is difficult to efficiently lower the Cr6+ content of the processed substances to an environmentally acceptable level.
Where chromium oxide-containing substances are reduced or incinerated according to the methods noted above, and the thus-processed substances are recycled in roadbeds, fillers in civil engineering reclamation works, temporary works and the like, they are checked as to whether or not chromium oxides were fully reduced, prior to being shipped. If the processed substances contain non-reduced chromium oxides, they cannot be recycled in roadbeds, fillers in civil engineering reclamation works, temporary works and the like.
An object of the present invention is to solve the problems in the prior art noted above, and to provide an industrial, simple and economical method for processing chromium oxide-containing substances, such as stainless steel slag, chromium slag, slag of molten wastes, sewage sludge, slag of molten sewage sludge, soil contaminated with chromium oxides and the like, thereby to completely prevent the release of Cr6+ from those chromium oxide-containing substances.
Another object of the invention is to provide a method for processing chromium oxide-containing substances such as those mentioned above within a short period of time without significantly increasing the volume of the substances processed to attain the intended results.
Still another object of the invention is to provide a method of recycling chromium oxide-containing substances, and to provide materials for roadbeds, fillers in civil engineering reclamation works, materials for temporary works, materials for civil engineering and construction works, etc.
Having carefully studied the problems in the prior art noted above, the present inventors have found that, in the prior art in which the defined reduction of chromium is attained in a high-temperature condition for melting, roasting or incineration, the reduction condition is difficult to control and the defined condition is not economical.
On the basis of this finding, we have tried methods of reducing Cr6+ at room temperature or at temperatures near room temperature. Specifically, we have made various studies and experiments for reacting various chromium oxide-containing substances such as those mentioned above with different aqueous solutions containing a variety of reducing agents, thereby to reduce Cr6+ in those substances.
As a result, we have found that, when water that contains sulfur having a valence less than 6, especially water that contains sulfur ions and sulfur as released from the non-aged material of blast furnace slag, is used, then chromium existing in chromium oxide-containing substances can be reduced with high reactivity in an industrial, simple and economical manner. On the basis of this finding, we have achieved the objects of the present invention.
In addition, based on this finding, we have further studied and have made the following findings [1] to [3]:
[1] Using any of sulfur-containing substances listed below as  less than 1 greater than  to  less than 4 greater than , the release of Cr6+ from chromium oxide-containing substances is extremely effectively prevented.
 less than 1 greater than  Water that has been used for cooling blast furnace
slag (hereinafter referred to as blast furnace slag-released water).
 less than 2 greater than  Slag containing sulfur and/or compounds of sulfur having a valence less than 6, for example, gradually-cooled blast furnace slag spontaneously aged for less than 3 months (hereinafter referred to as non-aged, gradually-cooled blast furnace slag), slag discharged from the pre-treatment of molten pig iron, etc.
 less than 3 greater than  Elemental sulfur, and substances containing elemental sulfur, such as sinter (flowers of sulfur) as recovered from sulfur-containing hot springs and the like (hereinafter referred to as elemental sulfur-containing substances).
 less than 4 greater than  Compounds of sulfur having a valence of less than 6, such as sodium thiosulfate, iron sulfide, hydrogen sulfide and the like (hereinafter referred to as compounds of sulfur with a valence of less than 6).
Generically, those substances  less than 1 greater than  to  less than 4 greater than  set forth above will be hereinafter referred to as sulfur-containing substances.
[2] Using any of those sulfur-containing substances  less than 1 greater than  to  less than 4 greater than  noted above, the release of Cr6+ from chromium oxide-containing substances is extremely effectively prevented according to any of the following processing methods (a) to (d).
(a) They are kept in an aerial atmosphere.
(b) Water is sprayed over them.
(c) Chromium oxide-containing substances are immersed in sulfur-containing substances.
(d) Steam is applied to them.
The above mentioned item (a) is a further surprising new finding. The release Cr6+ from chromium oxide-containing substances can be effectively prevented without using such aqueous solutions containing reduction agents as used in the first experiment described on page 6, 2nd and 3rd paragraph in this specification, according to the processing method of mixing the chromium oxide-containing substances with the sulfur-containing slag noted in [1]  less than 2 greater than  and then simply leaving the resulting mixture in an air atmosphere.
[3] Using the sulfur-containing substances  less than 1 greater than  to  less than 4 greater than  optionally combined with any divalent iron-containing substances (hereinafter referred to as Fe(II)-containing substances), such as ferrous sulfate, produces the following advantages (1) and (2).
(1) Even for blast furnace slag-released water having a low concentration of reducing sulfur, of which the concentration may vary in the water, any of the elemental sulfur-containing substances, the compounds of sulfur with a valence less than 6 or the Fe(II)-containing substances may be added thereto or sprayed thereover, and chromium oxide-containing substances to be processed are immersed in the resulting blast furnace slag-released water, whereby Cr6+ in the chromium oxide-containing substances can be reduced within a short period of time, resulting in that the release of Cr6+ from the thus-processed chromium oxide-containing substances is completely prevented.
(2) Even for slag that releases a large amount of Cr6+ according to the metal release test as set forth in Notification No. 46 of the Environment Agency of Japan, or for slag having a low degree of porosity, the chromium oxide-containing substances in the slag of that type may be mixed with any of the elemental sulfur-containing substances, and steam may be applied thereto, whereby the slag can be processed within a short period of time without increasing the volume of the processed slag, resulting in that the release of Cr6+ from the chromium oxide-containing substances in the thus-processed slag is completely prevented.
As preferred examples of the sulfur-containing substances having an overall sulfur content of not less than 10 % by weight for sulfur and/or compounds of sulfur having a valence less than 6, there can be mentioned elemental sulfur; elemental sulfur-containing substances such as sinter recovered from sulfur-containing hot springs, etc.; and compounds of sulfur with a valence less than 6 such as sodium thiosulfate, iron sulfide, hydrogen sulfide, etc. These may combined for use in the invention.
Where blast furnace slag-released water is sprayed over chromium oxide-containing substances to be processed therewith, the thus-sprayed substances are preferably kept in an aerial atmosphere to complete the reduction processing.
Also preferably, steam is applied to the chromium oxide-containing substances to be processed according to the invention prior to the intended reduction processing of the substances.
From 0.1 to 90 parts by weight of sulfur-containing slag may be added to and mixed with 100 parts by weight of the reduced, chromium oxide-containing substances to give materials for roadbeds, fillers in civil engineering reclamation works, materials for temporary works, and materials for civil engineering and construction works.